A conventional radio communication apparatus is explained using a document “Performance of SIR-Based Transmit Power Control using Outer Loop in the forward Link of DS-CDMA (TECHNICAL REPORT OF IEICE AP96-148, EMCJ96-83, RCS96-162, MW96-188 (1997-02).” This document describes a transmission power control method in CDMA. The following is an explanation of this description.
In transmission power control, measurement of SIR indicating the reception quality and increment/decrement of transmission power are performed at every slot cycle (0.625 ms). In this case, if the measured SIR is greater than the target SIR a command to reduce transmission power is sent to the base station (transmission side) and if the measured values is smaller than the target SIR a command to increase transmission power is sent to the base station. The base station increments or decrements transmission power according to this command.
Furthermore, the base station controls the outer loop taking into account the fact that the target SIR to acquire the required quality (FER: Frame Error Rate) varies depending on the environment of a mobile station. To be more specific, FER is measured from decoded data. This FER is compared with the target FER in every several frames and if the measured value is greater, the target SIR is increased and if the measured value is smaller, the target SIR is reduced.
The prior art performs transmission power control not only by sending a transmission power control command to the transmitting side based on the SIR measured by the mobile station but also by changing the target SIR through outer loop control.
However, the prior art has the following problem. That is, the target SIR increases depending on the environment and transmission rate of the mobile station and the reception SIR sometimes decreases due to fading, etc. In such a case, the mobile station instructs the base station to increase transmission power to make the reception SIR come closer to the target SIR, considerably increasing transmission power of the base station to the mobile station, which is likely to increase interference with other mobile stations to an intolerable degree.